This invention relates to a radar system, and, more particularly, to a radar system in which the waveform of the transmitted radar signal is determined according to chaotic coding.
In a radar system, a transmitter generates a transmitted radar wave which is propagated through free space. If the transmitted radar wave strikes an object, a portion of its energy is scattered and reflected from the object. A radar receiver has an antenna that receives a portion of the reflected energy. The received signal is analyzed to determine some characteristic of the object that reflected the radar wave, such as its distance or speed.
In many types of radar systems, the transmitted radar signal is produced by encoding a carrier signal with a code such as a binary code. The code may be encoded onto the frequency, phase, amplitude, or other characteristic of the carrier signal. The coding is carried by the transmitted and received radar signals, and the correlation of the received code with the transmitted code serves as the basis for obtaining information about the object that reflects the radar wave.
For example, if the wave form phase of the transmitted radar signal is encoded with a time series of a known type of code, that same code is correlated to the wave form of the received radar signal to determine the time interval between transmission and receipt of the radar signal. The time interval translates directly to a distance from the transmitter to the object and thence to the receiver, since the propagation velocity of the radar signal is at the constant speed of light. In the common case where the radar transmitter and receiver are at the same location, the radar signal propagation distance is twice the distance between the transmitter/receiver and the target.
Great care is taken in selecting the form of the code used to encode the radar signal. In many cases, the code is a repetitive sequence of individual binary pulses that is readily analyzed by the radar receiver. The rate of repetition of the sequence limits the maximum range of the radar, because at greater ranges there can be uncertainty as to which repetition is being matched to the received signal. The repetition rate of the binary pulses within the sequence determines the ranging resolution of the radar, because the receiver cannot resolve distances less than the distance associated with one binary pulse width. The number and character of the binary pulses within each sequence, prior to repetition, determines in part the ability of the radar to unambiguously correlate the transmitted and received signals. Finally, the nature of the coding may be important in minimizing the ability of countermeasures to negate the effectiveness of the radar system, a concern for military radars.
Thus, in selecting the coding used in a radar system, there is a constant tension between these various considerations. To obtain good ranging resolution of the radar, the pulse width of one bit of the code should be small. But to obtain good maximum range, there should be a low code sequence repetition rate. There is therefore a need for a radar utilizing a long coding sequence that is readily correlated between the transmitted and received radar signals.
Many different types of coding have been developed, studied, and used in an attempt to optimize the performance of radar systems. While these codings are operable in various situations, there is still a need for improvement in radar system performance by selection of improved signal coding. The present invention fulfills this need, and further provides related advantages.